An airstrike in Yemen this week killed dozens of children traveling on a bus on a field trip. It's the latest tragic development in the years-long war that has become one of the world's worst humanitarian crises. The strike came from the Saudi Arabia-led faction in the war - that's the same side the U.S. backs - in their fight against the local Houthis, who are viewed as proxies for Iran. Lise Grande is the humanitarian coordinator for the United Nations in Yemen. She joins me on the line from Amman, Jordan.

Lise, thank you for being with us. Could you just explain at this point what you have been able to learn about this particular attack?

LISE GRANDE: What we understand about the tragedy yesterday is that an airstrike hit a heavily populated market area. There was a school bus that had dozens of children that was directly impacted by the strike. We know that many children have died, and many more are wounded. It's a completely horrible and unacceptable tragedy. And the United Nations is now calling for an immediate, transparent, comprehensive investigation to determine the facts of what happened.

MARTIN: This is far from the first time that civilians have died in airstrikes by the Saudi-led coalition. I mean, what is their overall record? They maintain that this is not a war crime.

GRANDE: All of the parties to a conflict are obliged under international humanitarian law to do absolutely everything possible to ensure that civilians survive and that they have access to the assistance that they are entitled to. There are core bedrock principles that underlie humanitarian law. This includes the principles of distinction, precaution and proportionality. It is the responsibility of belligerents to ensure that they adhere to those principles - the principle of distinction, making sure that the target is not a civilian target; proportionality, making sure that whatever military gain they might have from the strike, it does not outweigh the civilian cost; and very importantly, the principle of precaution - belligerents have to make sure before there is a strike that whatever they are hitting is not civilian in character.

MARTIN: How are you getting aid in at this point? And what are the most critical needs?

GRANDE: Yeah. The humanitarian crisis in Yemen, as you said, is the worst in the world. Twenty-two million innocent civilians require some form of humanitarian assistance or protection. There is no other country in the world where 75 percent of the entire population needs help. The humanitarian agencies that are working in Yemen are doing everything we can to bring assistance in. We bring food in, health care. We're supporting water systems, sanitation systems. We're providing emergency kits. We're trying to support the victims of violence.

The humanitarians will do everything we can to try and help the civilians who need assistance in order to survive. But we can't solve the problem. The people who have started the war, they're the ones that need to solve it.

MARTIN: Lise Grande is humanitarian coordinator for the United Nations in Yemen. We spoke with her from her base in Amman, Jordan.